Coherent control of Bloch oscillations in semiconductor superlattices by means of optical pulse shaping

Riccardo Fanciulli, Purdue University

Abstract

The Bloch oscillations are a natural effect that may be observed when an electron is subject to a periodic potential (period d) and to a constant electric field, F. This effect is introduced in a semiclassical picture, first, and then in the quantum picture of beatings among localized wavefunctions corresponding to equally-spaced levels. An introduction to the techniques of observation of the Bloch oscillations is included, with special focus on the degenerate four wave mixing (DFWM) technique employed in our experiments. We did not observe directly the THz waveform from the sample, but used an indirect observation through DFWM to estimate the internal THz field and follow its change over time. The main focus of our work is the application of the laser pulse shaping technique to the Bloch oscillations. Laser pulse shaping is a new technique that allows to take a broad-band laser pulse and apply an amplitude and/or phase modulation to its spectrum. In our work the emphasis was placed on the creation of a tunable THz oscillator. We created, with our pulse shaper in amplitude modulation mode, a pulse that shares the same time period as the expected Bloch oscillations. This creates a kicked-oscillator kind of dynamics and makes the BO observable for up to 15 oscillations. Once the prolonged internal THz field was created, with phase modulation we achieved a switch for the oscillator that was capable to turn on and off the internal THz field. We then showed full control over the electronic dynamics by reversing the initial conditions of the electronic motion. This effect was again achieved by phase modulating the spectrum of the pump pulse. We then studied the presence of an optically created internal DC field in the superlattice structure. The effects of this DC field on the THz oscillator are discussed (broadening of the line, chirp). Finally, while doubling the frequency of our electronic oscillator, we observed an interesting effect of the DFWM where a transition that was not excited by the pump pulse ([special characters omitted]), upon arrival of the wide-band probe ([special characters omitted]), was nonetheless emitting a signal in the direction [special characters omitted].

Degree

Ph.D.

Advisors

Weiner, Purdue University.

Subject Area

Condensation|Optics

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